Current projection photolithography systems typically include an automatic focusing and leveling sub-system for accurately measuring a level and a tilt of the surface of an object. FIG. 1 is a schematic illustration of such a focusing and leveling system used in a conventional projection photolithography tool. As shown in FIG. 1, the focusing and leveling system includes a projection objective 19 and an optical measuring path extending on both sides of an optical axis of the projection objective 19. Disposed in the optical measuring path are an illuminating unit, a projecting unit, a detecting unit and a relay lens group arranged in this order. The illuminating unit is composed of an illumination light source 1, an illumination lens group 2 and fiber optics (not shown). Light emanated from the light source 1 is condensed by the illumination lens group 2 and directed through the fiber optics onto the projecting unit, thereby providing illuminating light to the whole measuring system. The projecting unit is composed of projection slits 3, an upstream projection lens group 4, a reflector group 5 and a downstream projection lens group 6. Light exiting the illuminating unit passes sequentially through the projection slits 3, the upstream projection lens group 4, the reflector group 5 and the downstream projection lens group 6, and then forms light spots to be measured in an exposure area on the surface of the object 7 being measured. The detecting unit is composed of an upstream detection lens group 8, a reflector group 9 and a downstream detection lens group 10. The relaying unit is disposed downstream relative to the detecting unit and is made up of a relay reflector 11, a relay lens group 12, a detector 13, a focusing controller 14 and an arithmetic unit 15. When travelling in the relaying unit, light forming the spots is received by the detector 13 and converted thereby to a light intensity signal indicative of the position and tilt of the surface of the object being measured.
As disclosed in one of its embodiments, the U.S. Pat. No. 5,414,515 provides a device with a single optical path and multiple light spots, in which, in order to achieve a measurement field of view having a required size, a plurality of charge-coupled devices (CCDs) is used. This design is disadvantageous in that the use of multiple CCDs, as wells as, corresponding image capture cards and cables, leads to high cost and increased footprint constraints on the whole photolithography tool in which the device is employed.
The Chinese patent CN101710291 discloses a solar energy device having a lens for refracting parallel light beams. This device differs from the subject matter of the present invention in terms of its application. CN101710291 focuses on the convergence of light beam energy and has no requirement on image quality. In contrast, the present invention is applied to imaging position adjustment and has requirements on resolution, distortion, focal depth and other image quality metrics. Therefore, there are essential differences between CN101710291 and the present invention.